1. Field of the Invention
This invention relates to a printing apparatus which performs printing by thermal transfer of ink on an ink ribbon to a printing object by the use of a thermal head, and a method of making a mask pattern for exposure by the use of the printing apparatus.
2. Prior Art
Conventionally, there has been proposed a printing apparatus of this kind which applies a strobe pulse to a thermal head at a predetermined voltage over a predetermined time period to thereby generate heat on the surface of the thermal head in an amount proportional to the product of the predetermined voltage and the predetermined time period, and applies the heat to the ink ribbon to effect thermal transfer of ink on the ink ribbon to a printing object with a sufficient adhesion.
In the conventional printing apparatus, the adhesion of ink to the printing object takes precedence over other factors, so that strobe pulses are applied to the thermal head at such a voltage and over such a time period that a sufficient adhesion of ink to the printing object is ensured. In other words, each strobe pulse is applied in such a manner that a sufficient quantity of heat is secured even if the printer is not under favorable operating conditions, e.g. when the thermal head is newly driven for printing or when the ambient (environment) temperature is low.
When the operating conditions of the printer are favorable for printing, the above settings of the strobe pulse application can bring about the inconvenience of an excessive rise in the surface temperature of the thermal head or generation of an excessively large quantity of heat, which results in irreversible thermal deformations, such as wrinkles, formed on the printing object and the ink ribbon. As a result, when the conventional printing apparatus is used to form a mask pattern for use in exposure, which is formed of a negative image of a printed image by thermally transferring a portion of ink coated on a transparent ribbon tape as a base material of an ink ribbon to a printing object, revealing a portion of the transparent ribbon tape corresponding to the printed image, there can be produced thermal deformations on the tape due to the excessively large quantity of heat, so that the negative image is deformed, and hence no longer suitable for the mask pattern for exposures